In my previous post, I asked: What are the performance implications of the two options?

Versus:

And the answer is quite simple. The chance to optimize how it works.

In the first example, we have to return an unknown amount of information. In the second example, we know how much data we need to return. That means that we can optimize ourselves based on that.

What do I mean by that?

Look at the method signatures, those requires us to scan a secondary index, and get the results back. From there, we need to get back to the actual data. If we knew what the size of the data that we need to return is, we could fetch just the locations from the index, then optimize our disk access pattern to take advantage of sequential reads.

In the first example, we have to assume that every read is the last read. Callers may request one item, or 25 or 713, so we don’t really have a way to optimize things. The moment that we have the amount that the caller wants, things change.

We can scan the index to get just actual position of the document on disk, and then load the documents from the disk based on the optimal access pattern in terms of disk access. It is a very small change, but it allowed us to make a big optimization.

I still don't understand. If you are streaming data, just get the first batch and wait until they have consumed enough the trigger the next batch. You don't need to know how many batches will be fetched in total since you are only reading them on demand.

If you are not streaming data, then you shouldn't be using IEnumerable.